1. Field of the Invention
The invention relates to a heating device of the light irradiation type in which a plate-shaped article to be treated, especially a semiconductor wafer, is subjected to rapid heating, holding at a high temperature, and rapid cooling for layer formation, diffusion, annealing or for similar purposes.
2. Description of Related Art
Heat treatment of the light irradiation type in the production of semiconductors, such as layer formation, diffusion, annealing or the like, in carried out in diverse ways. In each of these treatments, a semiconductor wafer (hereinafter also called only a wafer) which is a plate-shaped article to be treated or the like, is heated to a high temperature. If heat treatment of the light irradiation type is used for this heat treatment, the wafer can be quickly heated and its temperature raised in a few seconds to a few dozen seconds to at least 1000° C. When light irradiation is stopped, it can be quickly cooled. However, when a nonuniformity of the temperature distribution occurs in the semiconductor wafer, especially at a temperature of at least 1050° C., a phenomenon called slip occurs in the wafer, i.e., a defect of crystal transition. In this connection, there is the danger that scrap will be formed. Therefore, it is necessary in heat treatment of the wafer using a heating device of the light irradiation type to carry out heating, holding at a high temperature and cooling such that the temperature distribution of the wafer is made uniform. In the case of heating of the wafer for layer formation, to form a layer with a uniform thickness, the wafer must likewise be heated such that the temperature distribution of the wafer is made uniform.
Even if the entire surface of the wafer is irradiated with a uniform irradiance, while in a heating device of the light irradiation type the wafer is kept in the high temperature state, there are cases in which the temperature of the peripheral region of the wafer is low and the temperature distribution of the wafer becomes nonuniform, and in which the above described slip occurs. The reason for the temperature drop of the peripheral wafer region is that heat is radiated from the side of the wafer. The heat radiation from the wafer side causes in the wafer a heat flow, by which a temperature distribution is formed. In order to make the temperature uniform over the entire surface of the wafer, therefore the peripheral region of the wafer must be irradiated with light with a greater irradiance than in the center region of the wafer according to the amount of temperature drop as a result of heat radiation from the wafer side.
On the other hand, as one of the processes for preventing a temperature drop in the peripheral region of the wafer, ordinarily a process was proposed in which auxiliary material with the same heat capacity as the wafer is arranged such that the outer periphery of the wafer is surrounded by it. This auxiliary material is generally called a guard ring.
A case is imagined in which the wafer and guard ring are heated together with uniform light irradiation by the wafer and guard ring being regarded as a one-piece, individual virtual plate body. In this case, no heat radiation arises from the wafer side since the peripheral region of the wafer does not become the peripheral region of the above described virtual plate body. The temperature of the peripheral region of the wafer therefore does not drop. Since the guard ring is arranged such that the outside periphery of the wafer is surrounded by it, it can also act as wafer holding material when it acquires an additional means with which the peripheral edge region of the wafer is held fast. Therefore, there are many cases in which the guard ring is designed to hold the wafer fast. This means that the guard ring is a component designed to equalize the temperature drop which occurs due to heat radiation from the side of the wafer or from its vicinity, and to make the wafer temperature uniform. There are also many cases in which the guard ring is used as a wafer holding material.
However, in fact, it is difficult to produce the guard ring such that it can be considered integral with the wafer, i.e., that the heat capacities are the same. The reasons for this are the following:
1. It is very hard to produce the guard ring from the same material as the wafer. If the guard ring is produced from silicon (Si), which is the same material as the wafer, the heat capacities of the wafer and the guard ring can be made the same. Processing of silicon into a form with which the wafer is held fast is however very difficult. If it is repeatedly exposed to a large temperature difference, it deforms, by which it can no longer be used as a guard ring.
2. A material which can be processed relatively easily and which has a heat capacity with a value somewhat greater than silicon, but close to it, is silicon carbide (SiC) which is generally used for guard rings. However, since the thickness of silicon carbide cannot be made smaller than 1 mm due to processing problems (yield), it is greater than the wafer thickness of 0.7 mm to 0.8 mm.
3. Due to the difference between the specific heat of the above described silicon and the specific heat of the silicon carbide and to the difference between the thicknesses, the heat capacity of the guard ring when heated to a high temperature is roughly 1.5 times greater per unit of area than that of the wafer. In order to eliminate the above described difference between the heat capacities of the wafer and the guard ring, it is therefore necessary to irradiate the guard ring with light with a greater irradiance than the wafer.
The guard ring itself which holds the wafer is, on the other hand, supported in the heat treatment space of the heating device of the light irradiation type by a support which consists of silica glass or a ceramic such as silicon carbide or the like and is located in a metallic chamber as a component of the heat treatment space. The support generally supports the outer peripheral region of the guard ring. Since the support arrangement of the guard ring is made in the above described manner, in the outer peripheral region of the guard ring in addition to heat radiation from the side via the support, heat moves from the guard ring to the metallic chamber. Even if the guard ring is irradiated with light with a uniform irradiance, the temperature therefore drops in the outer peripheral region of the guard ring, causing a temperature distribution in the guard ring.
As was described above, a guard ring of silicon carbide or the like is produced with a smaller thickness. There are therefore cases in which the guard ring is damaged in the case of formation of stresses. If a stress which arises as a result of the temperature distribution in the above described outer peripheral region of the guard ring has exceeded an allowable value, the guard ring is damaged. If the guard ring is irradiated with light under the conditions of irradiance under which there is the possibility of damage of the guard ring by this stress, it is necessary to make the irradiance on the surface of the outer peripheral region of the guard ring greater than the irradiance on the surface at a point outside of the outer peripheral region of the guard ring and to reduce the deviation of the temperature distribution of the guard ring.
This means that both when using a guard ring and also when not using it, the outer peripheral region of the article to be treated (including the single virtual plate body in which the wafer and guard ring are formed integrally with one another) must be irradiated with light with a greater irradiance than the center region of the article to be treated.
FIGS. 1(a) & 1(b) each show an ideal distribution of the irradiance for making the wafer temperature uniform in light irradiation. FIG. 1(a) is a schematic of an ideal irradiance in the case of not using the guard ring. FIG. 1(b) is a schematic of an ideal irradiance in the case of using the guard ring. The x-axis is the distance from the center of the wafer. The central axes in these figures represent the center region of the wafer. The y-axis plots the relative values of the irradiance with which the surfaces of the wafer and the guard ring are irradiated. To simplify understanding, it is assumed that the distribution of the emissivity of the wafer surface is uniform. That is, it is assumed that the temperature of the wafer irradiated with light is proportional to the irradiance on the wafer surface.
As is shown in FIG. 1(a), for an ideal distribution of the irradiance in the case of not using a guard ring, the irradiance on the surface of the center region of the wafer is uniform. Furthermore the irradiance on the surface of the outer peripheral region of the wafer is greater than the irradiance on the surface of the center region of the wafer. This is used to equalize the temperature reduction by heat radiation from the edge face of the wafer.
On the other hand, as is shown in FIG. 1(b), for an ideal distribution of the irradiance in the case of using a guard ring, the irradiance over the entire surface of the wafer is uniform since the guard ring suppresses the effect of heat radiation on the outer peripheral region of the wafer. The irradiance on the surface of the guard ring is greater than the irradiance on the wafer surface. In particular, the irradiance on the surface of the outer peripheral region of the wafer is greater than the irradiance on the surface of the guard ring except for the above described outer peripheral region.
The irradiance on the surface of the guard ring according to FIG. 1(b) is greater than the irradiance on the surface of the outer peripheral region of the wafer according to FIG. 1(a). It is used to equalize the amount by which the heat capacity of the guard ring is greater than the heat capacity of the wafer, as was described above.
The reason for the greater irradiance on the surface of the outer peripheral region of the guard ring than the irradiance on the surface of the guard ring except for the above described outer peripheral region is the suppression of the effect of the temperature drop which occurs in the above described outer peripheral region of the guard ring. Under the conditions of light irradiation under which the guard ring is not damaged as a result of the temperature decrease which occurs in the above described outer peripheral region of the guard ring, i.e., in the case in which the stress which is caused by irradiation is less than or equal to an allowable value, the distribution of the irradiation of the guard ring can also be uniform.
FIG. 2 is a schematic of the arrangement of one example of a prior art heating device of the light irradiation type. FIG. 3 is a schematic of the positional relationships of the filament lamps, the wafer and the guard ring relative to one another for a heating device of the light irradiation type as shown in FIG. 2. In this connection, the heating device of the light irradiation type is viewed from overhead.
As is shown in these drawings, the heating device 100′ of the light irradiation type has a chamber 300′, a silica glass window 4′, a guard ring 5′, a light source part 10′ as the heating means and the like. The chamber 300′ and the silica glass window 4′ form a heat treatment space S2′ in which there is a wafer 6′. The guard ring 5′ is made of a ceramic material, such as silicon carbide or the like. In its inner edge area, the guard ring has a device 50′ which holds the wafer 6′ fast at the outer peripheral region of the wafer 6′. The holding device 50′ for holding the wafer 6′ which is located in the guard ring 5′ is shown, for example, in Japanese patent disclosure document 2000-58471 and corresponding U.S. Pat. No. 6,163,648. The guard ring 5′ is supported in the outer peripheral region 51′ of the guard ring by a support 9′ in the heat treatment space S2′. The lower end of the support 9′ is in contact with the chamber 300′.
The light source part 10′ is made such that several filament lamps 1A′ to 1S′ in the form of a rod-shaped bulb are arranged parallel to one another. For the respective filament lamp 1′, normally there is one filament inside. Because the respective filament in the filament lamps 1A′ to 1S′ receives power supplied by the feed devices 19A-1′, 19B-1′, . . . 19S-1′ which comprise the current source part 7′, radiant energy is emitted. The radiant energy emitted from the filament lamps 1A′ to 1S′ is delivered directly or by a reflector 2′ through the silica glass window 4′ into the heat treatment space S2′, and heats the wafer 6′ and the guard ring 5′ which are located in the heat treatment space S2′.
However, the conventional heating device of the light irradiation type has the following disadvantage.
As was described above, it is necessary to heat the guard ring 5′ with a greater irradiance than the wafer 6′, in order to make the temperature of the wafer 6′ uniform in the heat treatment of the wafer 6′. However, for the heating device 100′ of the light irradiation type, as shown in FIG. 3, the filaments of the filament lamps 1′ to 1Q′ which are arranged with respect to the wafer 6′ are arranged spanning both the wafer 6′ and also the guard ring 5′.
On the other hand, as was described above, there is one filament in each of the respective filament lamps 1A′ to 1S′. The irradiances which are radiated by the filaments to which power was supplied onto the guard ring 5′ and the wafer 6′ are therefore identical to one another. In the case of heating the wafer 6′ with a given temperature, therefore also the guard ring 5′ is heated with the same irradiance as that of the wafer 6′. This means that using a conventional heating device of the light irradiation type, adjustment and heating cannot be achieved such that the irradiance on the surface of the guard ring 5′ is greater than the irradiance on the surface of the wafer 6′.
A process for eliminating the above described disadvantage has been proposed in Japanese patent application 2005-191222 and corresponding commonly-owned, co-pending U.S. Patent Application Publication 2006-0197454 of which the present inventors are co-inventors. FIG. 4 schematically shows the positional relationship between the filament lamps, the wafer constituting the article to be treated, and the guard ring in a heating device of the light irradiation type which was described in this application. In this connection, the heating device of the light irradiation type is viewed from overhead.
This heating device of the light irradiation type, beside the filament lamp formed by the light sources, and besides the current source parts, has the same arrangement as the heating device of the light irradiation type shown in FIG. 2. In the light source part 10′ filament lamps in the form of rod-shaped tubes are arranged parallel to one another such that the distance between the respective central axes is the same. The respective filament lamp proposed in this prior application is characterized in that there are several filaments in the bulb which can be supplied individually. The specific arrangement of the filament lamps is shown in FIG. 13.
In FIG. 4, the filament lamps 1A′, 1B′ . . . 1S′ are arranged in the same manner as in the heating device of the light irradiation type shown in FIG. 2 in the space above the wafer 6′ and the guard ring 5′. The number and length of several filaments which are located in the respective filament lamps 1A′, 1B′, . . . 1S′ are fixed in this connection according to the shape of the wafer 6′ and of the guard ring 5′.
The number of filaments in the respective filament lamps 1A′, 1B′, . . . 1S′ is fixed as follows. In the filament lamps 1F′ and 1N′ which belong to the filament lamp group U1′ which is located in the center region of the light source part 10′, there are five filaments arranged such that there is one filament corresponding to the wafer 6′, two filaments corresponding to the inner peripheral region 52′ of the guard ring and two filaments corresponding to the outer peripheral region 51′ of the guard ring.
Furthermore, in the filament lamps 1C′ to 1E′ and 1O′ to 1Q′ which belong to the filament lamp groups U2′ which are located on the two outer sides of the filament lamp group U1′, three filaments at a time are arranged such that there is one filament corresponding to the inner peripheral region 52′ of the guard ring and two filaments corresponding to the outer peripheral region 51′ of the guard ring.
In the filament lamps 1A′, 1B′, 1R′, 1S′ which belong to the filament lamp groups U3′ which are located on the two outer sides of the filament lamp groups U2′, there is one filament according to the outer peripheral region 51′ of the guard ring.
The length of several filaments for the respective filament lamps 1A′, 1B′, . . . 1S′ is established as follows.
In the filament lamps 1F′ to 1N′ which belong to the filament lamp group U1′, the respective length of the respective filaments 1F-3′ to 1N-3′ which are located over the wafer 6′ is established such that the contour formed by the connection of their ends assumes a shape similar to the outer periphery of the wafer 6′.
Furthermore, the length of the respective filaments 1C-2′ to 1Q-2′ and 1F-4′ to 1N-4′ which are located in the filament lamps 1C′ to 1Q′ which belong to the filament lamp groups U1′, U2′ over the inner peripheral region 52′ of the guard ring is established such that the contour formed by the connection of their ends assumes a shape similar to the outer periphery of the inner peripheral region 52′ of the guard ring and the outer periphery of the wafer 6′.
Furthermore, the length of the respective filaments 1A-1′ to 1S-1′, 1C-3′ to 1E-3′, 1O-3′ to 1Q-3′ and 1F-5′ to 1N-5′ which are located in the filament lamps 1A′ to 1S′ which belong to the filament lamp groups U1′, U2′, U3′ over the outer peripheral region 51′ of the guard ring is established such that the contour formed by the connection of their ends assumes a shape similar to the outer periphery of the outer peripheral region 51′ of the guard ring and the outer periphery of the inner peripheral region 52″ of the guard ring.
The filament lamps 1A′, 1B′, . . . 1S′ shown in FIG. 4 have several filaments which can be supplied individually. The number and length of the above described several filaments are fixed each according to the shape of the wafer 6′, of the inner peripheral region 52′ of the guard ring and the outer peripheral region 51′ of the guard ring.
Therefore, it is possible to fix the radiant energy from the respective filament which is located over the guard ring 5′ to be greater than the radiant energy from the respective filament which is located over the wafer 6′. Furthermore, it is possible to fix the radiant energy from the respective filament which is located over the outer peripheral region 51′ of the guard ring to be greater than the radiant energy from the respective filament which is located over the inner peripheral region 52′ of the guard ring.
This makes it possible to establish a distribution of the illuminance which is near the ideal distribution of the illuminance shown in FIG. 1(b) on the surface of the wafer and the guard ring. Thus, heating of the light irradiation type can be done such that the temperature of the wafer 6′ is made uniform. However, since in the heating device of the light irradiation type shown in FIG. 4 several filaments which are located in the filament lamps 1A′, 1B′, . . . 1S′ are supplied individually, feed devices are required with a number corresponding to the number of filaments which are located in the bulbs of the filament lamps 1A, 1B′, . . . 1S′. When the number of feed devices increases, the current source part becomes larger and costs are increased.
To reduce the costs of the heating device of the light irradiation type, the number of filament lamps which form the light source parts can be easily reduced. However, when simply the number of filament lamps is reduced, the distribution of the irradiance on the irradiation surface becomes nonuniform.
FIGS. 5(a) to 5(c) each schematically show the relation between the number of filament lamps and the distribution of the irradiance on the irradiation surface. It is apparent therefrom that the uniformity of the irradiance distribution on the irradiation surface for a reduced number of filament lamps as shown in FIG. 5(b) compared to FIG. 5(a) is reduced. Especially in the case of heating of the article to be treated which is a silicon wafer at a temperature of at least 1050° C. does the disadvantage of the above described formation of a defect of crystal transition occur when the irradiance for the wafer becomes nonuniform.